This is not a show you watch for plot, relationships, or even real situational comedy in the classical sense. It is more equivocal to watching a stand-up comedian: it's all about the jokes and one-liners. But what it does, it does extremely well.
The comedic delivery of Ryan Reynolds, Nathan Fillion and the other primary cast members are what really make the show. Despite getting into cliched and predictable situations, the characters are lovable and the quotes are very memorable. It's too bad the writers didn't do better in the original storyline department, but in the end it doesn't even seem to matter so much because the comedy makes up for it.
As a side note, I would like to mention that, other than the Simpsons, this show has the most creative and hilarious Halloween specials I have ever seen. Really great stuff.